1. Field of the Invention
The present invention relates to solar-powered aircraft. More particularly, the present invention relates to configurations for arranging solar panels on a tail portion of an aircraft to optimize collection of solar energy to fully or partially power the aircraft.
2. Related Art
The advantages of pointable solar panels for solar-powered aircraft have long been recognized. While some configurations (e.g., NASA CR-3699) have panels separate from the normal stabilizing or lifting surfaces of the aircraft, there are weight and drag advantages to installing the panels on surfaces that are already needed by the aircraft. The normal wings and horizontal tails work best with the sun nearly overhead. However, these horizontal panels are of little use for a solar powered aircraft that can operate at higher latitudes in the winter time, where the sun elevation angle may only be 10 to 20 degrees above horizontal, even at noon.
It is possible to place panels on the vertical tail of an aircraft, but this surface is of relatively small total area, and is ineffective at higher sun elevations.
A cruciform arrangement of flying surfaces with solar cells on one side of one surface can track the elevation of the sun perfectly with no additional drag penalty. An early concept with cruciform wings was shown by Phillips in U.S. Pat. No. 4,415,133, the entire contents of which are incorporated herein by reference. A similar arrangement can be implemented as a reduced size version for the tail of the aircraft, as shown in U.S. patent application Ser. No. 12/211,027, the entire contents of which are incorporated herein by reference. With appropriate sizing of the tail moment arms, it is possible to have tail surfaces where the horizontal stabilizer area and vertical stabilizer area requirements are similar. Thus a cruciform tail with equal size surfaces can meet the aerodynamic requirements with little or no penalty in mass and drag.
Previous concepts for these sun-tracking tails have been cruciform with all surfaces of equal size. This allows for mass balancing about the roll pivot axis, minimizing the loads on the tail rotation actuator. With good balance it is even possible to “fly” the tails in roll by using small ailerons at the tips of some of the surfaces.
However, while the size of a tail is much smaller than a wing, solar-powered aircraft tails may still be quite large. A typically sized tail may have a span of as much as 50 meters. Therefore, a landing gear having a height of 25 meters would be needed for a conventional takeoff and landing. This height would cause a very large penalty in mass and/or aerodynamic drag. Thus, while prior art has shown the advantages of a sun tracking tail, there are major disadvantages arising from the large height of the tail during take off and landing. In addition, only about one-quarter of the wetted area of the tail may be covered with effective solar cells. The present invention is intended to address these shortcomings.